The utility of high quality thin film materials for various applications are well known in the art. As a reference, see "Deposition Technologies for Films and Coatings", by Rointon F. Bunshah, et al., 1982, Noyes Publications, Park Ridge, N.Y. Jersey, or "Thin Films for Free Atoms and Particles", edited by Kenneth J. Klabunde, 1985, Academic Press Inc., New York. There are now several processes used to prepare high quality thin film materials.
The Chemical Vapor Deposition (CVD) technique produces a non-volatile solid film on a substrate by the surface pyrolized reaction of gaseous reagents that contain the desired film constituents. A typical CVD process comprises the following steps, (1) gaseous reagent and inert carrier gas are introduced into the reaction chamber, (2) gaseous reagent is transported by convection and diffusion to the surface of the substrate, (3) reagent species are absorbed onto the substrate where they undergo migration and film forming reactions and (4) gaseous byproducts of the reaction and unused reagents are removed from the chamber. The pressure in the deposition chamber may be atmospheric or reduced as low as a fraction of 1 torr, as in the respective cases of Atmospheric Pressure CVD (APCVD) or Low Pressure CVD (LPCVD). The energy required to drive the reactions is supplied as heat to the substrate. For practical reaction rates, substrates are typically heated to temperatures ranging from 500.degree. C. to as high as 1600.degree. C. Consequently, heat sensitive substrates cannot be processed in this manner.
Energy can also be supplied by an RF electric field which powers a gas discharge in the deposition chamber near the substrate surface. This process is referred to as Plasma Enhanced CVD (PECVD). In PECVD, the substrate temperature may be 300.degree. C. or lower. However, the substrate is immersed in the discharge which can also lead to plasma damage of the substrate and the film during growth.
The CVD deposition rate also depends on the local concentration of the gaseous reagent near the substrate surface. Increasing reagent partial pressures can lead to higher film deposition rates. When local reagent concentration is too high, however, undesirable reaction and nucleation of solid particles in the gas phase can occur. These particles can then precipitate onto the substrate surface where they contaminate the growing film. This is especially true for PECVD. It is always desirable to develop methods of film deposition which occur at lower temperatures and which avoid problems associated with plasma damage and gas phas nucleation of particles. In addition, it is desirable to have methods which avoid diffusional mass transport limitations, as film deposition may be limited.
Downstream CVD processing involves reaction of RF or microwave plasma-generated oxygen, or nitrogen radicals with silane or other CVD reagent gas, wherein the CVD reagent gas is introduce into the reaction chamber downstream of the plasma. (See e.g. "Deposition of Device Quality Silicon Dioxide Thin Films by Remote Plasma Enhanced Chemical Vapor Deposition", S. S. Kim, D. V. Tsu and G. Lucovsky, J. of Vac. Sci. & Tech. A 6(3), 1740-4.)
Physical Vapor Deposition (PVD) includes the methods of evaporation (metallizing), sputtering, molecular beam epitaxy, and vapor phase epitaxy. These processes typically occur in a chamber evacuated to below 10-6 torr. The desired film material is present in the chamber as bulk solid material. The material is converted from the condensed phase to the vapor phase using thermal energy (i.e. evaporation) or momentum transfer (i.e. sputtering). The vapor atoms or molecules travel line-of-sight as free molecular rays across the chamber in all directions where they condense on prepared substrates (and on the chamber walls) as a thin film. If the pressure becomes too high, collisions with gas molecules interfere with the vapor transport which therefore reduces the deposition rate. Sputtering can also cause undesirable plasma damage to the thin film and substrate.
Reactive evaporation and sputtering processes involve the intentional introduction into the chamber of oxygen, nitrogen or other reactive gas in order to form oxide, nitride or other compound thin films. Reactive gas pressure must be limited as mentioned above in order to avoid interfering with the transport of the depositing vapor. When the pressure is too high, undesirable nucleation of particles in the gas phase can occur. In conventional reactive processes the solid source material can be contaminated by unwanted reaction with the reactive gas.
It is therefore desirable to develop a method and apparatus of thin film deposition which is operable at higher pressure without diffusion governed transport limitations. It is also desirable to have a method and apparatus of reactive thin film deposition which can occur at a high rate without contamination of a gaseous reagent source. The present invention is directed toward such a method and apparatus.